1. Technical Field
The present disclosure relates to a nitride semiconductor structure having a principal plane that is a semi-polar plane, an electronic device including the nitride semiconductor structure, a light-emitting device including the nitride semiconductor structure, and a method for producing the nitride semiconductor structure.
2. Description of the Related Art
Nitride semiconductors have been used as materials of various semiconductor devices such as optical devices (e.g., white light-emitting diode (LED) and semiconductor laser), power devices (e.g., field effect transistor (FET)), and electronic devices.
The stable crystal structure of the nitride semiconductor is a hexagonal wurtzite structure. Therefore, nearly all of practical optoelectronic devices employ a nitride semiconductor structure in which the +c-plane of the hexagonal crystal structure serves as a principal plane.
FIGS. 1A and 1B illustrate the crystal structure of a nitride semiconductor having a wurtzite structure. In FIG. 1B, the notations “a1”, “a2”, “a3”, and “c-axis” denote the orientation of crystallographic axes, and the notations “+c-plane” and “m-plane” denote the orientation of crystallographic planes. The +c-plane (0001) is a crystallographic plane whose normal is the c-axis. The nitride semiconductor does not have inversion symmetry in the c-axis direction. That is, the crystal of the nitride semiconductor has a front side and a back side in the c-axis direction. A high-quality crystal is more likely to be grown in the +c-plane than in other orientations. Accordingly, in general, optoelectronic devices have been produced by stacking a nitride semiconductor structure having a principal plane that is the +c-plane on, for example, a sapphire substrate having a principal plane that is the c-plane in order to produce LEDs, on a silicon (Si) substrate having a principal plane that is the (111) plane in order to produce electronic devices, or a gallium nitride (GaN) bulk substrate having a principal plane that is the +c-plane in order to produce semiconductor lasers.
As described above, relatively high-quality crystals are likely to be grown when the principal plane of the nitride semiconductor is the +c-plane. Therefore, the +c-plane of a nitride semiconductor has been commonly used as a principal plane in order to produce a nitride-semiconductor-based optoelectronic device or to grow a high-quality GaN bulk substrate.
However, it is also known that a nitride semiconductor multilayer structure in which the +c-plane serves as a growth surface has a characteristic due to the asymmetry of the crystal structure of the nitride semiconductor in the c-axis direction. Specifically, polarization occurs in such a nitride semiconductor multilayer structure. The asymmetry of the crystal structure of the nitride semiconductor in the c-axis direction is due to the imbalance in the distribution of cationic gallium (Ga) atoms and anionic nitrogen (N) atoms as illustrated in FIG. 1A. The imbalance in the distribution of these ions causes the polarization to occur in the c-axis direction.
There are two types of polarization: spontaneous polarization and piezoelectronic polarization. In particular, occurrence of piezoelectronic polarization has a strong correlation with the strain generated in the crystal. For example, in an indium gallium nitride (InGaN) well layer stacked on a GaN layer, the higher the indium (In) content in the InGaN well layer, the larger the amount of strain that occurs in the crystal and the greater the degree of polarization that occurs in the InGaN well layer. The polarization causes an internal electric field to occur in the c-axis direction in the InGaN quantum well constituting an active layer. When the internal electric field (i.e., mainly piezoelectric field) occurs in the active layer, a position gap between the distribution of electrons and the distribution of the holes in the active layer may be created due to the quantum confined Stark effect of carriers, which reduces internal quantum efficiency. This leads to an increase in the threshold current of a semiconductor laser, an increase in the power consumption of an LED, and a reduction in the luminous efficiency of an LED. In addition, screening of the piezoelectric field may occur with an increase in the density of the injected carriers, which causes emission wavelength to vary.
As described above, crystal growth on the principal plane that is the +c-plane facilitates formation of a high-quality crystal structure, but may also cause adverse effects due to spontaneous polarization and piezoelectric polarization. Hereinafter, the c-plane, which is a plane such that polarization occurs when crystal grown is performed on the plane, is referred to as “polar plane”.
In order to address the above-described issues, there have been proposed a nitride semiconductor crystal structure having a principal plane other than the +c-plane and a method for growing such a nitride semiconductor crystal structure. For example, growing a nitride semiconductor in the a-axis direction or the m-axis direction as illustrated in FIG. 1B prevents the above-described polarization from occurring.
When crystal growth is performed in the a-axis or m-axis direction, that is, when the a-plane or the m-plane serves as a principal plane, polarization does not occur in the direction of the normal to the plane because, in these planes, Ga atoms and N atoms lie in the same atomic plane. Thus, if a semiconductor multilayer structure is formed using, for example, the m-plane as a principal plane, a piezoelectric field would not be formed. For example, LEDs having such a non-polar plane as a principal plane have a higher luminous efficiency than LEDs of the related art which have a principal plane that is the c-plane. Hereinafter, a crystallographic plane in which polarization does not occur is referred to as “non-polar plane”, and the term “m-plane” collectively refers to the (1-100) plane, the (−1010) plane, the (10-10) plane, the (−1100) plane, the (01-10) plane, and the (0-110) plane. As described above, polarization occurs in a nitride semiconductor crystal structure having a principal plane that is the polar c-plane, but not in a nitride semiconductor crystal structure having a principal plane that is the non-polar a-plane or m-plane.
It is described in Japanese Journal of Applied Physics Vol. 39 (2000), pp. 413-416 that, even when the principal plane of the nitride semiconductor crystal structure is not the non-polar a-plane nor m-plane, the degree of polarization becomes small compared with the case where the principal plane of a nitride semiconductor crystal structure is the polar c-plane. It is described in Japanese Journal of Applied Physics Vol. 39 (2000), pp. 413-416 that, when an In0.1 Ga0.9N/GaN multilayer structure having a thickness of 3 nm is formed on a principal plane that is the (11-24) plane or the (10-12) plane, the degree of polarization can be reduced to a level comparable to that to which the degree of polarization can be reduced when the nitride semiconductor crystal structure has a principal plane that is a non-polar plane. The (11-24) plane refers to a plane whose normal is inclined at 39 degrees with respect to the c-axis in the a-axis direction, and the (10-12) plane refers to a plane whose normal is inclined at 43 degrees with respect to the c-axis in the m-axis direction. Hereinafter, planes inclined with respect to the polar c-plane and the non-polar a-plane or m-plane are collectively referred to as “semi-polar plane”.
As described above, it is known that a nitride semiconductor crystal structure having a principal plane that is a non-polar plane or a semi-polar plane is capable of reducing the degree of polarization, which occurs in a nitride semiconductor crystal structure having a principal plane that is the polar c-plane.
However, in general, it is difficult to perform crystal growth for forming the nitride semiconductor structure having a principal plane that is a non-polar plane or a semi-polar plane compared with a structure of the related art having a principal plane that is the polar +c-plane. Thus, it is difficult to reduce the amount of crystallographic defects in order to obtain high-quality crystals.
Japanese Patent Application No. 2008-551465 discloses a method in which a nitride semiconductor thin-film having a principal plane that is a semi-polar plane is formed on an intentionally miscut substrate (i.e., inclined substrate). It is described that the density of crystallographic defects can be reduced by growing a nitride semiconductor structure having a principal plane that is a semi-polar plane on a substrate intentionally inclined with respect to a low-index crystallographic orientation while controlling the direction and angle of the inclination of the substrate appropriately, and thereby crystallinity can be improved. According to Japanese Patent Application No. 2008-551465, the inclination angle is desirably 0.5 degrees to 20 degrees and is more desirably 0.5 degrees or more and 3.0 degrees or less. The results of forming a nitride semiconductor having a principal plane that is the semi-polar (11-22) plane on an m-plane sapphire substrate having a principal plane that is the (1-100) plane are described in Examples of Japanese Patent Application No. 2008-551465. In this study, the direction and angle of the inclination of the m-plane sapphire substrate were changed within the range of −0.5 to 1.0 degrees with respect to the m-axis in the c-axis direction. It is described that increasing the inclination angle improves crystallinity, which is evaluated in accordance with the half width measured by X-ray diffraction in the (11-22) plane of the nitride semiconductor, and that setting the inclination angle to 1.0 degrees improves crystallinity best. However, a case where the inclination angle exceeds 1 degree is not described in Examples of Japanese Patent Application No. 2008-551465.